A problem that some people have been having is that external drives that used to mount on the Apple Mac, no longer mount and are inaccessible. It often happens after an upgrade or update.
Why should an external drive that used to work with your Mac no longer work? You cannot mount it or access the files it holds. Is this a bug in macOS? No, it is not a bug. Sometimes Apple changes the way macOS works and it can affect the software you use and the hardware devices you plug in.
Show drives on the desktop and Finder
You probably know this already if you have been using external drives with your Mac, but I’ll mention it anyway. Show drives on the desktop and in the sidebar to make them easy to access and to make it obvious when they have or have not mounted.
Open Finder > Settings and select the General tab. Tick the options to show hard disks and external disks on the desktop. Select the Sidebar and select them in the Locations section.
If an external drive does not mount, what can you do? Here are some suggestions.
Check the connections
This probably isn’t the cause of the problem, and we will see the most likely cause in a minute, but the first thing to do is to check the connections. Does the external drive need power? If it does, check that it is plugged in, and the power is on.
Unplug the drive from the Mac and plug it back in to make sure it has a good connection. Try a different port on the Mac. If there are no free ports, try swapping the drive with something else you have plugged in. If you have a spare cable, try it. I would not go as far as buying a new one because it is unlikely to be the problem.
Update software and extensions
A common reason why an external drive will not mount on the Mac is because it uses a file format that is not supported. But it worked before! Maybe it did, but the format may not be supported anymore.
A drive may be formatted using the NTFS file format which is used by Windows PCs, while the natural format for Macs is APFS. It is possible to add software to the Mac to enable it to read and write NTFS drives, such as Microsoft NTFS for Mac by Paragon. An external drive that has been used on a Linux PC may be formatted using extFS, and Paragon has software that enables the Mac to read and write it.
There are other suppliers of software to enable a Mac to read and write non-Mac format drives, and if you have it, a macOS update or upgrade could make it incompatible. You might not realize you are relying on it until a drive does not mount.
If you have any software or extensions that do anything related to disks, drives, partitioning, formatting and so on, check for updates either in the software or at the developer’s website. It may be needed.
Use a Mac file format for the drive
The best way to avoid problems with mounting external drives is to format them in a way that macOS can understand. This is best done when you first use a new drive and it is empty. If there are files on it, you must copy them off either by getting the drive to work on your Mac or by attaching it to another computer.
Assuming you are OK with wiping the drive, plug it in and open Disk Utility in the Utilities folder on the Mac.
Select the drive in the sidebar and click the Erase button in the toolbar. In the Scheme menu, select GUID Partition Map (source).
In the format menu, select APFS or APFS (Encrypted) depending on whether you want extra security. The case-sensitive option means Document and document are two different files. I would only choose case-sensitive if you are used to it and know what you are doing. Now go ahead and erase the drive.
If you need to be able to access the drive on a Windows PC, select ExFAT for the file format. APFS has advantages, but PCs cannot read it. If you need to use the drive on an old Mac running an old version of macOS, select Mac OS Extended (Journaled). APFS is better, but macOS 10.13 High Sierra or later is needed. See file formats in Disk Utility.